Tc. Wu et al., MONTMORILLONITE UNDER HIGH H2O PRESSURES - STABILITY OF HYDRATE PHASES, REHYDRATION HYSTERESIS, AND THE EFFECT OF INTERLAYER CATIONS, The American mineralogist, 82(1-2), 1997, pp. 69-78
Dehydration of Ca- and Mg-exchanged montmorillonite was studied along
H2O isochores in the hydrothermal diamond-anvil cell by in situ X-ray
diffraction using a synchrotron radiation source. At pressures between
the H2O liquid-vapor (L-V) boundary and similar to 10 kbar, the dehyd
ration temperature for the conversion from the 19 Angstrom hydration s
tate to the 15 Angstrom hydration state occurred over the temperature
range 260-350 degrees C for Ca-exchanged montmorillonite and 200-250 d
egrees C for Mg-exchanged montmorillonite, with a slight increase with
increasing pressure. For both materials, the rehydration from the 15
Angstrom to 19 Angstrom states occurred at the same temperature as deh
ydration at pressures along the H2O L-V boundary, thus showing no hyst
eresis. The rehydration hysteresis increased to nearly 75 degrees C at
6 kbar for Ca-exchanged montmorillonite and to the same amount at 2.5
kbar for Mg-exchanged montmorillonite. Dehydration experiments on Mg-
exchanged montmorillonite along the isochores of 1.024 and 0.75 g/cm(3
) showed conversion from the 15 Angstrom hydrate to the 12.5 Angstrom
hydrate at 590-605 degrees C. The 12.5 Angstrom hydrate only partially
rehydrated after cooling to room temperature along those two isochore
s. In an experiment started from the 15 Angstrom state, in which the p
ressure was below the H2O L-V curve, dehydration occurred at 400-500 d
egrees C and rehydration at 430-350 degrees C. When our previous resul
ts on Na-exchanged montmorillonite are combined with the current exper
imental data, systematic trends can be found in the effect of pressure
and interlayer-cation species on the dehydration temperature and rehy
dration hysteresis.